Hi all,
If it's above the water line? you should be all right, you need to make sure that the moss is one that is growing on the same substrate in the tank as it was in your garden, as some moss only grow on acid rocks, some on alkaline, some only on wet wood etc. although some species will grow on a wide range of substrates (damp soil etc.) Some of our moss is very attractive in a terrarium google "Thuidium tamariscinum" for a really common one in the UK.
For submerged you need to buy tropical aquatic mosses.
cheers Darrel